Under transverse structures are understood natural or artificial structures introduced into water transversely to the water bed, which have an influence on the natural flow conditions and also on the bottom or bank structures in the body of water. Examples of transverse structures are ground sills, bottom sills, support sills, sill ramps, sill steps, sill slides, supports, weirs or dams.
Migration fish bypasses are fish aids facilitating fish migration in streaming bodies of water, which make it possible for fish and other animals living in water to overcome obstacles such as for example weirs, or natural obstacles such as river bottom steps. The large differences between the water levels located on the head water and tail water side make it impossible for the fish to overcome these obstacles, for example by springing. It is known that in order to gradually reduce the distance between the water levels upstream and downstream, fish migration bypasses can be provided with several basins having different water levels.
However, at many sites with hydroelectric plants, a very large number of basins is necessary in order to meet the actual requirements for the height of the falling water. Together with the typical requirements on the size of the basin (for example, the length of the basin=three times the length of the fish with the usual fish length of about 1 meter), very large fish migration passes are created. Their construction then becomes so expensive that even in areas where free construction is possible, the construction at existing weirs is so expensive that an investment in renewable energy through hydropower is often not worth it.
In addition, constructions in such large water construction sites can be realized only at a very significant expense. The subsoils are unknown and proximity to old buildings of unknown qualities of the construction materials encountered in the location result in cost-driving surprises that are often a rule rather than an exception.
In a number of locations, large migration bypasses for fish cannot be build also on the grounds of monument protection as they would interfere with aspects of the old buildings located around the body water.
An embodiment of a bypass for fish migration (also called a fish bypass), which does not require much space and which can overcome a large gradient, is a fish lift. With a fish lift, fish which are put into a tub that is equipped with a substantially vertical shaft are lifted with a pulling device from the tail water to the head water. The tub is then tilted into the head water so that the water located in the tube is passed along with the fish into the head water. A disadvantage of fish lifts is that their design is structurally complex and that they require a great deal of energy, especially since the entire weight of the tub filled with water in which the fish are collected must be lifted up. In addition, attracting the fish so that they would swim into the tail water in the tube, and in particular tipping the tub over into the area of the head water, is not very gentle with respect to the fish. An additional disadvantage is that the fish can swim into the fish lift only when the tube is arranged at the very bottom of the shaft.